This patent application is an extension of the high impedance monobus technique disclosed in the concurrently filed, copending patent application of D. J. Morgan and D. C. Smith, Ser. No. 580,292 filed May 23, 1975, to provide lossless multiline and multistation conferencing in a key telephone system. In such a key telephone system, in order to utilize the existing system structure it is necessary to provide a conference circuit interposed between each key telephone line circuit and the associated stations and to replace the conventional hybrid in the stations with a similar conference circuit for the purpose of converting from two-wire operation to monobus operation. The monobus arrangement utilized an impedance element with is common to all of the stations connected to the conference. The purpose of the monobus technique is to reduce the transmission loss which results when several stations or lines are connected together in conference fashion.
Since in key telephone systems there is no common circuitry available other than the line circuits, it is necessary to equip each line conference circuit with the common monobus impedance element. When more than one line becomes connected to the conference, the impedance elements associated with each line become connected in parallel thereby changing the voltage signals on the bus. In addition to affecting the transmission levels of the system, such a change in voltage defeats the feedback concellation inherent in the monobus technique. Thus, a need exists in the art for a control arrangement designed to prevent more than one such monobus impedance element from becoming connected to the same conference.